Merged Transistor Logic (MTL) exhibits many appealing features such as excellent speed-power products, small cell size, low number of masks and diffusion steps. FIG. 1A shows a cross-sectional view of an MTL cell fabricated using a conventional bipolar process. The conventional bipolar process comprises the following steps. First, a low doped n-type epitaxial layer 7 is grown on an n.sup.+ surface which serves as a common emitter to all vertical NPNs. A guard ring 4 is then driven in, preferably deep enough to contact the n.sup.+ surface. Next, a p-type diffusion forms region 8 for the emitter of the lateral PNP device and region 10 for the collector and base of the PNP and vertical NPN, respectively. A final n.sup.+ diffusion is used to form the collectors of the vertical devices. Obviously, this process also yields conventional vertical NPN devices. The lateral PNP device is biased to function as a current source to drive the vertical NPN device and charge capacitances during switching transients. The electrical parameters of importance to the switching performance of the vertical device are its current gain and switching frequency, f.sub.T. For the lateral device, only beta is important since it is always in a conduction state. It is well known that the switching performance of MTL cells fabricated by conventional bipolar processing is limited by their typically low beta and f.sub.T owing to inappropriate doping profiles. A listing of the features sought in a high performance switching bipolar device will help to identify the inadequacies of the conventional MTL cells.
(1) The active collector area should be large relative to the active emitter area to minimize recombination in the extrinsic or inactive base region. PA1 (2) The ratio of emitter doping density to base doping should be large to insure that the current flowing across the emitter-base junction is mainly made up of one type of carriers; namely, the emitter majority carriers injected into the base. This leads to an injection efficiency close to unity and high beta. However, emitter doping larger than 5.multidot.10.sup.20 cm.sup.-3 is detrimental to high betas, as discussed by R. P. Mertens, H. J. DeMan and R. J. Van Overstraeten in "Calculation of the Emitter Efficiency of Bipolar Transistor," IEEE Trans. Elect. Dev., September 1973. PA1 (3) It is also shown in the above cited article by R. P. Mertens, et al., that compensation of emitter donor impurities by base acceptor impurities is also detrimental to obtaining high betas. PA1 (4) The emitter-base junction should have a steep doping profile like that of an abrupt junction to minimize minority carrier storage within the emitter junction and associated emitter storage capacitance and hence increase f.sub.T. PA1 (5) the base doping profile should decrease from emitter to collector so that an aiding electric field is set up which reduces the transit time of injected carriers across the base region for larger f.sub.T. PA1 (6) the base doping outside the active base region should be high to reduce the extrinsic base resistance, R.sub.B, and thereby minimize debiasing of the emitter-base junction and reduce RC time constants. PA1 (7) The collector doping should be high to reduce parasitic collector resistance and stop the widening of the base region at high current densities, which prematurely reduces f.sub.T as discussed by Kirk in "A Theory of Transistor Cutoff Frequency (f.sub.T) Fall-Off at High Current Density," IRE Trans. Electron Dev., 1962. The associated high collector capacitance is of no consequence for MTL circuits since they have no resistor collector load and their voltage swings are small. PA1 (1) For the vertical device, the area of collectors V.sub.01 and V.sub.02 is smaller than the area of the emitter 7 and consequently do not collect the minority carriers injected by the emitter sections located between collectors which are lost through recombination. This recombination current effectively diminishes the vertical beta. Since the use of a single emitter 7 contributes to the high packing density of MTL, a reversal of this unfavorable area ratio is not feasible. PA1 (2) The epitaxial layer 7 forming the emitter junction of the vertical device is lightly doped so that its injection efficiency is low and storage capacitance is high. In the article by F. M. Klaassen, "Device Physics of Integrated Injection Logic", IEEE Trans. Elect. Dev., March 1975, formulas have been derived for the vertical beta and f.sub.T which show that the epitaxial layer 7 should be thin and highly doped for best performance. PA1 (3) The epitaxial layer 7 doping is compensated by the diffusion of the base region 10 doping impurities which is detrimental to the injection efficiency according to the aforementioned article by R. P. Mertens, et al. PA1 (4) The doping of the extrinsic base region of the vertical device is the same as and determined by the doping of its active base region. Since the latter must be relatively low in order to have high injection efficiency, R.sub.B is generally high. PA1 (5) The doping of the emitter 8 of the lateral device is not in the optimum 10.sup.20 cm.sup.-3 range since it is fabricated simultaneously with the base 10 of the vertical device to a doping level in the 10.sup.17 cm.sup.-3 range dictated by the requirements of high vertical beta and compensation of the doping impurities of the base 10 by the subsequent diffusion of vertical collectors V.sub.01 and V.sub.02. PA1 (6) the base 10 doping profile of the vertical device increases from emitter 7 to collectors V.sub.01 and V.sub.02, just the opposite of the fifth feature listed above.
After axamination of the structure of FIG. 1A, it becomes apparent that both its vertical and lateral devices possess few of the features listed above. The following deficiencies are noted:
Moreover, the cell of FIG. 1A suffers from significant recombination losses in its non-active regions which decrease the beta of the lateral device. The n.sup.+ guard ring 4 reduces the loss of holes injected through the outer sidewall 5 of the lateral emitter 8. However, hole injection through the bottom p-n.sup.- junction significantly reduces the lateral beta. Hole losses also take place through surface recombination since the injection efficiency of the emitter 8 is best near the surface where its doping concentration peaks. The leakage current from these hole losses decrease the base current drive and the number of vertical devices that can be driven from a single lateral PNP with a resulting decrease in circuit density. These leakage currents also increase the power dissipation of the cell.